1. Field of the Invention
This invention relates to rotary polygon mirror devices, and more particularly to a rotary polygon mirror device which employs a dynamic pressure air bearing for rotating its rotary polygon mirror.
2. Description of the Related Art
For instance in a laser beam printer, the laser beam must be accurately deflected at high speed. In order to meet this requirement, a rotary polygon mirror is employed. A rotary polygon mirror is in the form of a regular polyhedron with mirrors on its sides. The rotary polygon mirror is fixedly mounted on the rotor of a drive motor, so that it is rotated at high speed. Therefore, a rotary polygon mirror rotated at a speed of lower than 15,000 rpm employs a ball bearing, and a rotary polygon mirror rotated at a speed higher than 15,000 rpm employs a fluid bearing utilizing air or magnetic fluid, or a fluid bearing in combination with a magnetic bearing (cf. Japanese Patent Application Publication No. 6854/1978, and Japanese Patent Application (OPI) No. 164413/1984 (the term "OPI" as used herein means an "unexamined published application")).
In one example of the fluid bearing, herring bone or spiral grooves are formed in the surface of the rotor or in the surface of a member confronted through a small gap with the rotor, and a fluid drawing phenomenon due to the rotation of the rotor or the confronted member is utilized to produce a high pressure. In another example, such as a movable pad type fluid bearing, a plurality of pads are arranged around the rotor which can be freely tilted, in such a manner that small gaps are locally formed, and high dynamic pressures produced there are utilized.
In any of the bearing systems, the rotation in vibration cannot be obtained unless the gap is maintained most suitable. Of those bearing systems, the dynamic pressure groove system has a small range of tolerable gap dimension, several micro-meters (.mu.m) to several tens of micro-meters (.mu.m). Hence, in forming the bearing, it is necessary to use a material such as a ceramic or a special alloy which is not highly affected in dimension by thermal variation and is wear resistant. However, the use of such material provides another problem that it is difficult to form spiral grooves or the like in the component of the bearing. This increases the manufacturing cost.
Furthermore, in the case where, in order to increase the bearing rigidity, a viscous fluid other than air is employed in the dynamic pressure groove system, the polygon mirror is limited in range of speeds. That is, when the polygon mirror is turned at high speed, a windage loss or bearing loss occurs to increase the temperature, as a result of which the bearing characteristic becomes unstable, and therefore the allowable range of speeds of the polygon mirror is limited.
The movable pad type air bearing operates stably over a relatively wide range of temperature variations. However, it is intricate in construction and accordingly high in manufacturing cost.
FIG. 1 is a sectional view of a conventional polygon mirror rotating motor. As shown in FIG. 1, a polygon mirror 105 and a rotor 106 are mounted on a rotating body 104. When current is applied to a stator 107, the rotor 106 is rotated in the direction of the arrow A. As the rotating body turns, it draws the air around it, so that the rotating body 104 and upper and lower bearings 101 are spaced from each other. Each of the upper and lower bearings 101 comprises: a bearing base 102; and a wear-resisting plastic member 103 bonded to the bearing base 102.
The shaft of the motor is held vertical, and its thrust end supporting the whole weight of the rotating body 104 is floated by the force of repulsion induced between two permanent magnets 108 and 109 which are set with the same poles.
Thus, the motor is in a non-contact state both in the radial direction and in the thrust direction. Therefore, when the motor is turned at high speed, it should rotate smoothly; however, in practice, it vibrates. There are some causes for the vibration of the motor. One of the causes is the imbalance of the rotating body. This cause may be eliminated by detecting the imbalance of the rotating body 104 with a balance tester. Another cause of the vibration is the bearings. In this case, the vibration cannot be eliminated with the tester.
More specifically, the vibration may attributed to the fact that the upper and lower bearings 101 are not coaxial, or to the configuration of those bearings 101. In the case where the upper and lower bearings 101 are not coaxial, the vibration occurs as follows: That is, the rotating body 104 is turned with its rotating axis tilted because of the misalignment of the bearings 101, so that the gap between the rotating body 104 and the bearings 101 becomes non-uniform, thus causing the vibration. In order to eliminate the misalignment of the bearings 101, heretofore the following method is employed: That is, the position of one of the bearings 101 is adjusted with screws in three directions until it aligns with the other bearing 101.
The vibration attributing to the configuration of the bearings 101 is called "whirl vibration". It has been considered that the whirl vibration can be eliminated by using a bearing 101 having an inner surface which is made up of a plurality of circular-arc surfaces as shown in FIG. 2 or 3. However, in practice, the vibration cannot be eliminated even with such a bearing. This will be described in more detail.
As shown in FIG. 2 or 3, the gap between the rotating body 104 and the bearing 101 is gradually decreased in the direction of rotation. As the rotating body 104 is turned, the gas in the gap is drawn viscously by the relative movement of the surfaces; that is, the gas is pushed in the gap, thus producing a pressure (or positive pressure) to float the rotating body 104. Thereafter, the gap is gradually increased in the direction of rotation of the rotating body 104. In this case, the viscosity of the gas produces a pressure (or negative pressure) to pull the rotating body 104. As a result, while the rotating axis rotates with an angular speed .omega., the rotating body 104 turns around the center of the bearing 101 in the direction of rotation of the rotating axis with a radius corresponding to an amount of eccentricity e and with a swirling angular speed .omega..sub.0. The swirling angular speed .omega..sub.0 is 1/2 to 1/3 of the angular speed .omega..
Furthermore, the vibration may be caused when the gap between the rotating body 104 and the bearings 101 is changed with temperature. This is due to the fact that the rotating body 104 is different from the bearings 101 in thermal expansion coefficient. This will be described in more detail below.
Heretofore, the bearing base 102 is made of copper or plastic material, and the rotating body 104 is made of iron or steel. When the rotating body 104 is turned, the temperature is increased, and, since the thermal expansion coefficient of the bearing base 102 is higher, the inside diameter of the latter is increased by thermal expansion more than the diameter of the rotating body 104, so that the gap therebetween is increased. Hence, the dynamic pressure is decreased while the rigidity is lowered, so that vibration occurs.
On the other hand, if, in the case where the wear-resisting plastic member on the bearing base 102 is relatively thick, the plastic member is smaller in thermal expansion coefficient than the bearing base 102, then the gap is increased similarly as in the above-described case. However, when the plastic member is equal to or larger than the bearing base 102 in thermal expansion coefficient, then the gap is decreased, so that the bearing loss is increased, and the temperature rises greatly.
As was described above, the conventional dynamic pressure bearing is disadvantageous in that it will vibrate the rotating body 104 unstably. The vibration attributing to the misalignment of the two bearings 101 can be eliminate by adjusting the positions of the bearings 101 so that they are coaxial with each other. However, this method provides another problem that the adjustment required time and labor, and the number of components is increased.
When the speed of rotation is increased, the energy loss is increased: that is, the temperature rises, so that the bearing gap is varied, with the result that the vibration is produced. Hence, it is difficult to increase the speed of rotation to a high value (20,000 rpm or higher). In order to overcome this difficulty, it is necessary to externally cool the bearing 101, and accordingly the motor.